The present invention relates to a hydraulic control system for an automatic transmission.
Automatic transmissions are known wherein one friction unit is engaged and another friction unit is disengaged to effect a shifting between speed ratios, i.e., between drive ratios. In operating such automatic transmissions, if the engagement of the one friction unit does not take place in good timed relationship with the disengagement of another friction unit, substantial shocks will take place or an engine will be raced. Particularly, in the case where the same friction unit must be actuated not only during an upshifting but also during a downshifting, it is necessary to actuate the friction unit in different timings optimum for the upshifting and downshifting, respectively. In a known four speed automatic transmission, a brake must be released in timed relationship with the engagement of a first clutch during 2-3 shifting, while the same brake must be released in timed relationship with the engagement of a second clutch during 4-3 shifting. For optimum shifting, it is desirable to supply fluid pressure chambers of the brake and the first clutch with fluid pressure via a common orifice for 2-3 shifting and to supply fluid chambers of the same brake and the second clutch with fluid pressure via another common orifice for 4-3 shifting. This is because it is preferred that the engagement of the clutch and the release of the brake take place almost at the same time so as to provide the appropriate engagement and release timing by supplying the oil pressure via the common orifice.